Steam is the most widely used thermal mass, and it is an important two energy. It is widely used in electric power, smelting, steel, automobile, textile, printing and dyeing, chemical fiber, food, medicine and other fields. As an important energy source of various industries, accurate measurement of steam flow is of great significance to save energy and improve the economic efficiency of enterprises.

The influence of steam temperature and pressure changes into saturated steam and superheated steam, the steam flow measurement by the effect of density is relatively large, so the need for compensation of temperature and pressure in the measurement of steam flow (saturated steam can be single compensation for temperature or pressure).

At present, vortex flowmeter and differential pressure flowmeter are the main flow meters used in steam flow measurement. Because the differential pressure flowmeter has great pressure loss, large maintenance and low precision, it is gradually eliminated by users. Compared with the vortex flowmeter, it has obvious advantages in measuring steam flow.

Saturated steam metering system

The steam without overheating is called saturated vapor. It is colorless, odorless, non burning and non corrosive gas. The content of liquid droplets or fog in saturated steam reflects the quality of steam, which is generally expressed by the parameter of dryness. The dryness of a steam is the percentage of dry steam in a unit volume of saturated steam.

The saturated steam metering system has the following characteristics:

The temperature and pressure of saturated steam correspond to each other, and there is only one independent variable between the two.

Saturated steam is easy to condense, and if there is heat loss during transmission, it leads to the decrease of temperature and pressure. Strictly speaking, saturated steam contains more or less two-phase fluid with droplets or liquid fog, so different states can not be described by the same equation of gas state.

The accurate measurement of saturated steam flow is more difficult, because the saturated steam quality is difficult to guarantee, generally can not accurately detect double phase flow of fluid flow, the steam pressure fluctuation will cause the change of steam density, showing the value of additional error meter. Therefore, in the steam metering, we must try to keep the dryness of the steam at the measuring point to meet the requirements. If necessary, we should take compensation measures to achieve accurate measurement.

  

Superheated steam metering system

Superheated steam is a relatively special medium, and generally speaking, steam means superheated steam. Superheated steam is a common power source, which is often used to drive the turbine to rotate, and then to drive the generator or centrifugal compressor. Superheated steam is obtained by heating the saturated steam without any liquid droplets or liquid fog, which belongs to the actual gas. The temperature and pressure parameters of superheated steam are two independent parameters, and their density should be determined by these two parameters. It can be checked by the superheated steam density meter. After a long distance transport of superheated steam, with the conditions (such as temperature and pressure) changes, especially in the superheat is not high, because the heat loss and the temperature decreases from overheating into saturated or supersaturated, become saturated steam or with droplets of saturated steam. The saturated steam suddenly decreases greatly, and the liquid will expand into superheated steam when it is adiabatic. Thus, the vapor liquid two phase flow medium will be formed.

Measurement analysis

At present, the use of flow meters to measure steam flow, measuring medium refers to single-phase superheated steam or saturated steam. For the steam with constant phase change, there must be some inaccuracy in measurement. The solution to this problem is to keep the superheat steam, to reduce the moisture content of the steam, heat preservation measures such as strengthening the steam pipe, reducing the loss of pressure steam, in order to improve the accuracy of steam measurement. However, these methods can not completely solve the problem of inaccurate measurement of steam flow. The fundamental way to solve this problem is to develop a flow measuring instrument for measuring two-phase flow medium.

When the working state of the steam deviates from the design state, the flow indication will produce errors. It also affects the flow measurement, so the measurement of steam flow needs to be compensated, and the compensation factor is complicated because of the change of steam condition. Superheated steam density is determined by steam pressure and temperature of two different parameters, and the range of parameters, expressions of density is not the same, can not be expressed by the same formula, so cannot obtain the uniform density formula, only individual derived temperature and pressure compensation formula. In the range of temperature and pressure fluctuation, besides the temperature and pressure compensation, the expansion coefficient of gas should also be considered.

Regardless of the flowmeter detection of saturated steam flow in steam pressure fluctuation under the condition of pressure compensation measures must be taken, this is because in the flow equation, there are factors of steam density, working conditions and design conditions are not the same, the reading error will be generated, and the size of working pressure and design pressure deviation error, P >P will be negative error, otherwise there will be a positive error. Dry steam conditions is an important condition to accurate measurement of steam flow, currently being developed online steam quality measuring instrument, the degree of instrument applied to steam flow measurement and compensation system to be dry, will further improve the measurement accuracy. The following three measures should be taken at present:

(1) the pipeline conveying steam must have good insulation measures to prevent heat loss.

(2) in the steam pipeline to block water, at the lowest point of the pipeline and the instrument before the pipe should be equipped with traps, timely discharge condensate.

(3) boiler operation should avoid the phenomenon that the liquid level of drum is too high, so as to minimize the fluctuation of the load.

Selection and comparison of instrument for saturated steam flow

At present, with as many as 60 kinds of flow meters are mainly industrial, vortex flowmeter, differential pressure (Kong Ban, pitot tube, pipe) flowmeter, shunt type flowmeter, Annubar flowmeter, float type flowmeter. In history, there is no flow instrument suitable for any fluid, any range, any flow state, and any use condition. Any flow meter has its specific applicability and limitations. If the flow meter is not properly chosen, the flow must be measured incorrectly. But the flow measurement is a complicated technology, and many kinds of flow meters, even for a certain application, choose a suitable flow meter will become a highly technical work, need factors before making the final choice carefully and thoroughly consider and Quan Hengxu related measurement problems. Therefore, the correct choice of instrument is a key instrument in normal use, in the practical application, too much fault is due to instrument selection unreasonable, a detailed understanding of the field application of the operating conditions and the medium parameters, select the appropriate pressure, temperature, protection, explosion-proof grade and material structure, to ensure that the instrument can run in the best condition.

Steam metering should consider 5 main factors in the selection of flow meter: measurement methods, performance requirements and instrument specifications, measured fluid characteristics, environmental conditions, economic conditions (purchase costs, installation costs, operating costs, maintenance costs, expenses check). In our practical work, whether it is used for industrial and mining enterprises heating centralized production, is the most used orifice plate (nozzle) flowmeter and vortex flowmeter, based on the two meter as an example explained:

Differential pressure flowmeter

Differential pressure flowmeter is composed of a flow detection element is installed in the pipeline in (which produce a differential pressure device, referred to as the instrument of differential geometry), known fluid conditions and a meter and a pipeline to calculate the flow meter, a pressure device, pressure pipe and pressure gauge is composed of three parts. The orifice flow meter as the representative of the differential pressure flowmeter application has a long history, high degree of standardization, the application is widespread, may not be the real flow calibration, differential display instrument standardization and serialization and high level of generalization, the theory of high precision, wide application range and strong adaptability, low initial investment cost. But through practical application, it is found that the orifice flowmeter is also inadequate:

Many factors in the application (the design parameters and operating parameters are inconsistent, the upstream straight pipe length, orifice plate and pipe is not concentric orifice A surface contamination, sharp wear etc.) has great influence on the measurement accuracy, the measurement error increases, resulting in reduced accuracy. In particular, it is necessary to check the differential pressure loosening device to ensure the accuracy of the zero point, and check the three way valve regularly to prevent the measurement error caused by the blockage.

The installation quantity is big, the more trouble, and the demand is high, need regular maintenance and cleaning of the larger workload.

It needs to be equipped with differential pressure transmitter to increase maintenance work. Besides, the pressure pipe is needed, and the pressure pipe should be insulated in winter. It can not be installed outdoors.

The range ratio of flow is 1 to 3~1: 4: the range is low, the measurement of small flow is difficult, and the range of flow is narrow.

The pressure loss is larger, the scale is nonlinear, and the operation cost is high.

LGK series orifice plate flow meter, +P31 series differential pressure transmitter, +P21 series high temperature pressure transmitter, +XSR22 Chinese flow meter + armored thermal resistor (not for integration, for reference)

LGV series V cone flow meter, +P31 series differential pressure transmitter, +P21 series high temperature pressure transmitter, +XSR22 Chinese flow meter + armored thermal resistor (not used for integration, for reference)

ortex Flowmeter

Vortex flowmeter is a new type of flowmeter developed based on Carmen vortex street principle. From 70 to 80s, vortex flowmeter developed rapidly, and developed many types of vortex flowmeter with resistance and detection, and put on the market in large quantities. China's development has reached the peak in the street, dozens of, it should be said that the vortex flowmeter is in the development of products, but because it has the advantages of other flowmeter can not have both, a large proportion of amplitude of vortex flowmeter has been widely used in various fields of rise, will in the future flow meter in the dominant hole an ideal substitute of the flowmeter.

It has the following characteristics:

The structure of the utility model is simple and firm, and the measuring part has no movable parts, and the operation is reliable for a long time.

Maintenance is small, maintenance is very convenient and installation cost is low.

The output pulse is directly proportional to the flow rate, has zero drift, has high accuracy, and is convenient for networking with the computer.

Wide range of applications, applicable to various gas, steam and liquid flow measurement.

The flow measurement range is wide, and the range ratio can reach 1 to 10.

The small pressure loss, low operation cost, more energy saving.

In a certain range of Reynolds number, the frequency of the output signal is not affected by the fluid physical properties and component changes, the meter coefficient only with the shape and size of the vortex body, volume flow measurement of fluid without compensation, the coefficient of exchange parts without re calibration instrument.

But the flowmeter also has some limitations

Vortex flowmeter is a kind of velocity flowmeter. The stability of vortex separation is affected by the velocity of flow, so it has certain requirements for straight pipe section. Generally it is the front 10D and the back 5D.

When measuring liquid, the upper limit velocity, pressure loss and cavitation limit are generally (0.5-8) m/s.

When measuring the gas, the upper limit velocity is limited by the compressibility of the medium, the lower velocity is limited by the Reynolds number and the sensitivity of the sensor, and the steam is (8-25) m/s.

The stress vortex flowmeter is sensitive to vibration, so it is necessary to have some shock absorption measures when the flow rate of the pipeline is larger.

The stress vortex flowmeter adopts piezoelectric crystal as the detection sensor, so the temperature is limited to (-40-+350) DEG C, and the measuring temperature is not higher than 400 DEG C, otherwise, the probe is easily damaged due to aging.

The anti electromagnetic interference and radio frequency interference difference.

When the fluid medium for two-phase flow or pulsating flow on measuring effect.